Austenitic stainless steels have good corrosion resistance in many environmental conditions, but they have low hardness and poor friction and wear properties. Attempts have thus been made to develop surface treatment methods for improving these properties. However, surface modification of austenitic stainless steels usually has to overcome two major problems. One problem is the formation of an oxide scale (Cr.sub.2 O.sub.3) on the steel surface due to the strong affinity of chromium, which is the principal alloying element in austenitic stainless steels, with oxygen in air. This oxide scale frequently results in poor adhesion between a coating and the steel surface. Therefore, such surface modification techniques as PVD coatings, electroplating and electroless plating have limitations for stainless steels, as compared with coating and plating of most other ferrous alloys.
Another problem associated with surface treatment of austenitic stainless steels lies in the fact that in many cases the improvement in surface hardness and wear resistance of the steels by surface treatments is accompanied by a loss in the corrosion resistance. For example, plasma nitriding, which is carried out in a glow discharge in a nitrogen gas-containing mixture at a pressure of 100 to 1000 Pa (1 to 10 mbar), is one of the most widely used methods to treat stainless steel surfaces, resulting in a nitrogen diffusion layer having high hardness and excellent wear resistance. However, nitriding hardening is induced by the precipitation of chromium nitrides in the nitrided layer. This leads to a depletion of chromium in the austenite matrix and thus a significant reduction in corrosion resistance [see E. Rolinski, "Effect of Plasma Nitriding Temperature on Surface Properties of Austenitic Stainless Steel", Surface Engineering, Vol. 3, No. 1. 1987, pages 35-40].
Therefore, attempts have been made to develop surface treatment methods for improving the wear resistance of austenitic stainless steels without losing their corrosion resistance. A low temperature plasma nitriding technique has been developed, in which a conventional dc or pulsed plasma nitriding apparatus is used. The process is carried out at temperatures below 500.degree. C. for a time up to 60 hours in a nitrogen-containing gas of pressure 100 to 1000 Pa(1 to 10 mbar) [see P. A. Deamley, A. Namvar, G. G. A Hibberd and T. Bell, "Some Observations on Plasma Nitriding Austenitic Stainless Steel", Proceedings of the First International Conference on Plasma Surface Engineering, Garmisch-Partenkirchen, Germany, 1989, pages 219-226.] Low temperature nitriding can produce a nitrided layer having high hardness and good corrosion resistance. However, the hardened layer is very thin and brittle, and it is difficult to achieve uniform layer thickness.
A low pressure plasma carbon diffusion treatment has recently been proposed for stainless steels, in which a triode ion plating apparatus is used and the treatment is carried out at temperatures between 320.degree. C. and 350.degree. C. and in a gas mixture of argon, hydrogen and methane [see P. Stevenson, A. Leyland. M. Parkin and A. Matthews, "Effect of Process Parameters on the Plasma Carbon Diffusion Treatment of Stainless Steels at Low Pressure", Surface and Coatings Technology, Vol. 63, 1994, pages 135-143.] A working pressure of 1 to 2 Pa (0.01 to 0.02 mbar) is used for the treatment, which requires the use of a diffusion pump throughout the treatment lasting up to 30 hours. An additional sputter cleaning stage of several hours is required to effect carbon mass transfer and diffusion. A typical process comprises 4 hours sputter cleaning in argon or argon and hydrogen mixture, followed by 20 hours treatment at 320-350.degree. C., producing a carburised layer of 11 .mu.m thick with a maximum hardness about 7000 MN/mm.sup.2 (700 HV.sub.0.01). No corrosion test results are reported for this treatment The low pressure plasma carbon diffusion treatment uses an expensive and complicated triode ion plating system, and requires operation of the diffusion pump throughout the process and an additional sputter cleaning step. In addition, the growth rate and hardening response of the layer are low. Similar comments apply also to the procedures described in GB-A-2261227.
K. T. Rie et al (Haerterie-Technische Mitteilungen, vol 42, No. 6, Nov. 1, 1987, pages 338-343) disclose plasma nitriding and plasma nitrocarburising procedures conducted so as to produce a compound layer on sintered mild steel, which has a body-centred cubic structure. In contrast to this, the present invention is concerned with austenitic stainless steels which have a face-centred cubic structure.
Th. Lampe et al (Haerterie-Technische Mitteilungen vol 46, No. 5, September 1991, pages 308-316) disclose plasma nitriding and plasma nitrocarburising procedures conducted so as to form a compound layer on iron-based material such as sintered mild steel, ledeburitic cast iron and pearlitic cast iron which, like the mild steel of K. T. Rie et al (supra), has a body-centred cubic structure.
G. V. Shcherbedinskii et al (Metal Science and Heat Treatment, 34 (1992) May/June, Nos. 5/6, pages 375-378) also disclose a procedure for the plasma nitrocarburising of high speed steels using dicyanogen formed in situ by decomposition of ferrocyanides.